Under the influence of vertical shear, adiabatic vortices develop large tilts such that the upper and lower vortices become separated by up to several hundred kilometers. Diabatic vortices generally have much smaller tilts, usually much less than the radius of maximum winds. The diabatic heating associated with the convection develops a wave number 1 asymmetry that opposes the tilting and strongly couples the lower and upper vortices. The upward motion part of the asymmetry usually occurs downshear or slightly downshear left while the precipitation is typically downshear-left.

Recent studies have described the structure of Hurricane Bonnie on August 23, 1998 using multiple observations from the National Aeronautics and Space Administration's Convection And Moisture EXperiment (CAMEX-3). On that day, Bonnie's structure was highly asymmetric with maximum low-level reflectivities on the eastern side of the storm. Observations indicated isolated but deep convective towers that formed on the southeastern side of the storm and reached their maximum extent on the northern side. A conceptual model for the evolution of the convective towers suggested that the convective cells were initiated on the downshear side and grew to produce maximum precipitation on the downshear left side. This conceptual model further suggested that the precipitation development occurred as air parcels rose through the depth of the troposphere starting on the downshear side and ending on the upshear side, i.e., within half a revolution about the center.

This study examines a high-resolution simulation of Hurricane Bonnie using the Pennsylvania State University-National Center for Atmospheric Research mesoscale model (MM5). Attention will be given to mechanisms for generating asymmetric vertical motion and the impact of this asymmetry on air parcel motions and precipitation development within the eyewall. Air parcel trajectories calculated from three-minute model output suggest two basic modes of ascent within the eyewall. The first is associated with convective towers similar to those observed in Bonnie in which air parcels rise to the upper troposphere within half a revolution about the center. A second and much more frequent mode consists of air parcels rising from the boundary layer to the upper troposphere within two or more revolutions about the storm with the rising motion occurring predominantly on the downshear to downshear-left side of the storm. Vertical motion asymmetries producing these trajectories appear to be related to the direction of relative flow through the storm. The effects of relative flow and vortex tilt will be examined in greater detail for the conference.